This project seeks to determine the roles of cyclin-dependent kinases (CDKs) in endomitosis (defined as repeated DNA synthesis in the absence of cell division). This process occurs during megakaryocyte differentiation, and is modeled in the laboratory by phorbol ester treatment of human Erythroleukemia (HEL) cells. Endomitosis is correlated with an increase in cdk2 protein and a reduction in cdc2 protein. The proposed research will examine the reduction in cdc2 levels via studies of its half life and possible degradation. The role of each CDK will be assessed by the use of dominant negative mutants of cdk2 and cdc2, to see how these affect the endomitotic pathway. These studies will shed light on the regulation of S phase, and further our understanding of megakaryocyte development.